This work is based on the rationale that the functional significance of intracranial self-stimulation (ICSS) lies in the anatomical and pharmacological systems responsible for this behavior. To determine the nature of these systems, a 4-phase systematic analysis of predetermined ICSS loci has been initiated. These phases are: 1) mapping of specified brain regions for ICSS properties; 2) using neuroanatomical techniques to identify all of the neural systems in the vicinity of these ICSS sites. (Examples of such techniques are horseradish peroxidase, 3H-amino acid autoradiography, silver impregnation, and histochemical fluorescence.); 3) manipulating ICSS pharmacologically with the use of agents whose effects on ICSS have been demonstrated empirically (e.g., CNS stimulants, neuroleptics, narcotics); and 4) selectively lesioning, either specifically with 6-hydroxydopamine or else non-selectively with electrolytic current, the neural systems determined in earlier phases of the work to be possible contributors to ICSS from a given brain region. Data collected to date indicate that several of the systems that impinge upon, or pass through forebrain ICSS sites, originate in brainstem nuclei that are themselves capable of supporting ICSS. Previous results pointing to mediation of ICSS by catecholaminergic systems has been brought into question for certain midbrain ICSS regions, and corroborated for other midbrain ICSS regions. In the case of rejection of the catecholaminergic basis of ICSS, alternative systems have been identified anatomically, and are being analyzed for their functional contributions to this behavior.